Self-regulated dc to dc converter

ABSTRACT

A regulated power supply using square wave generator the pulse width of which is governed by an impedance controlled by the converter output. The impedance is impressed on the square wave generator through an insulating transformer with two separate secondary windings, one connected to the diode bridge output of an output voltage error detector and the other connected to the diode bridge output of an overcurrent and overvoltage protecting device; in this manner physical isolation is provided between the error detector, as well as the protecting device, and the square wave generator and thus between the output and input of the power supply. The square wave generator output operates on the control input of a separate power inverter which is also used to provide a regulated, but isolated, output for normally powering the square wave generator. Initially the square wave generator is powdered by a preregulator associated with the DC input to the converter. In addition to the conventional output pulses the square wave generator also furnishes quenching pulses to minimize the switching losses in the switching transistors of the power inverter.

United States Patent 51 May 2, 1972 Businelli [54] SELF-REGULATED DC TODC CONVERTER [72] inventor; Pietro Businelli, Milan, Italy [73]Assignee: GTE Automatic Electric Laboratories incorporated, Northiuke.ill.

[22] Filed: Dec. 7, 1970 [21] Appl. No.: 95,675

[30] Foreign Application Priority Data Mar. 25, 1970 Italy ..22408 152]U.S.Cl ...32l/2,321/18,321/27 MS [51] Int. Cl. ..H02m 3/28 [58] FieldoiSearch ..321/2, 16, 18, 27 MS [56] References Cited UNITED STATESPATENTS 3,205,424 9/1965 Bates ..321/27 MS 3,031.629 4/1962 Kadri3.219906 11/1965 Keller et a1. 3.297936 1/1967 Ruch i. 3.390.322 6/1968Rogers ..321/27 MS Primary Examiner-William M. Shoop, .Ir.Attorney-Cyril A. Krenzer and B. E Franz ABSTRACT A regulated powersupply using square wave generator the pulse width of which is governedby an impedance controlled by the converter output. The impedance isimpressed on the square wave generator through an insulating transformerwith two separate secondary windings, one connected to the diode bridgeoutput of an output voltage error detector and the other connected tothe diode bridge output of an overcurrent and overvoltage protectingdevice; in this manner physical isolation is provided between the errordetector. as well as the protecting device. and the square wavegenerator and thus between the output and input of the power supply. Thesquare wave generator output operates on the control input of a separatepower inverter which is also used to provide a regulated, but isolated.output for normally powering the square wave generator. initially thesquare wave generator is powdered by a preregulator associated with theDC input to the converter. In addition to the conventional output pulsesthe square wave generator also furnishes quenching pulses to minimizethe switching losses in the switching transistors of the power inverter.

9 Claims, 11 Drawing Figures Patented May 2, 1972 4 Sheets-Sheet 1 Fl IFRF l k K POWER RECTIFIER o FILTER INVERTER AND FILTER UNIT k I DA GOP.RE

AEE WA E sou v A STARTER 6 "mm R ERROR DEVICE E DETECTOR PROTECTIONDEVICE T v "W INVENTOR AGENT Patented May 2, 1972 4 Sheets-Sheet 2 I I II l I I I I I r J f w WT 4 sham-sum 8 as I Plhnted May 2, 1972 FIG 4FIG. 6a

4 Shun-Shut 4 Patunhd May 2, 1972 SELF-REGULATED DC TO DC CONVERTERBACKGROUND OF THE INVENTION l. Field of the Invention This inventionrelates in general to power supply circuits connected to DC distributionsystems, and more particularly to stabilized power supply devices. Theprincipal purpose of these devices is to provide a substantiallyconstant output voltage.

2. Description of the Prior Art A conventional voltage regulatorfunctions by way of a closed negative feedback loop. An error detectorcompares the output voltage across the load with a fixed referencevoltage and feeds any discrepancy in these two voltages to a regulatingdevice which is located between the input voltage source and the errordetector. The regulating device is normally a variable impedance or aswitch, electronic or otherwise, generally connected in series with theinput voltage source. Regulators of this type fulfill their purposereasonably well in that they stabilize the output voltage with respectto variations not only in input voltage but also in load.

Nevertheless these regulators are either rather complicated or theirefficiency is not always satisfactory. As they are normally not able toprovide galvanic isolation between the input source and the load,undesirable ground loops often result, with the danger of instabilityand shock hazard.

According to the present state of the art, self-regulated converters areof two types:

a. non-regulated converters preceded or followed by a voltagestabilizer.

b. converters with intrinsic regulation, generally operative to "chop,or vary the pulse width of the square wave present at the outputtransformer and subsequently rectify it in such a manner that an averageDC voltage proportional to the chopping" or pulse ratio is obtained. Nodescription is given of converters of type (a) as they are not relevantto the present invention.

For converters of type (b), relevant to the invention described herein,certain embodiments are known, among which are the following:

l. US. Pat. No. 3,295,043 issued Dec. 27, l966 to RP. Massey. Theself-regulated converter of the Massey patent operates according to thewave chopping principle by utilizing the rectangular hysteresis loopmagnetization characteristic of the power transformer. This circuitprovides the desired isolation between input and output circuits but itdoes not stabilize the output voltage against load variations.

2. U.S. Pat. No. 3,219,907 issued Nov. 23, 1965 to E.

Josephson, and US. Pat. No. 3,2l9,906 issued on the same date to C. H.Keller et a]. As in the case of (l) above, these patents make use of thecharacteristic of a rectangular hysteresis loop. However, they do notprovide the desired isolation between input and output circuits.

U.S. Pat. No. 3,031,629 issued Apr. 24, [962 to F.V. Kadri. This circuitalso uses wave chopping and operates by controlling the phasedisplacement between two multivibrators (inverters). The resultantvoltage of the two inverters, which is a chopped rectangular wave, isaveraged and filtered according to the usual methods and fed to theload. The circuit regulates both for load variations and for variationsof input voltage. However, the circuit shown in the Kadri patent doesnot provide isolation between output and input circuits. Further thereis no protection against overloads. Since the phase regulation actsdirectly on the power stage there is a more than negligible loss ofpower in the master inverter due to the saturation of the iron,especially when the commutation frequency is of the order of tens ofkilocycles, and the cost is also greater because the magnetic materialused in self-saturated inverters is generally of the rectangularhysteresis loop type.

The object of the present invention is to provide a self-regulated DC toDC converter which presents none of the disadvantages of conventionalconverters of this type, but has all their advantages. More particularlythe objects of the invention are:

l. to improve the efficiency of voltage regulation, even for variationsof input voltage greater than percent.

2. to provide the desired galvanic isolation between the input circuitand output circuit.

. to obtain an output impedance lower than that normally obtained by theconventional arrangement in which a voltage stabilizer is followed by anon-regulated converter.

4. to provide a plurality of output voltages of any desired polarity andmagnitude.

. to provide protection against output overcurrent which is sufficientlyrapid to protect the regulator semiconductors against overloads,regardless of which output voltage is involved.

6. to provide such protection in the form of a current limiter, that is,a circuit having an approximately constant current overloadcharacteristic, with the result, among others, that the supply isautomatically restored when the unstandard condition ceases to exist.

7. to insure for a given power and number of available voltages, and anequipment size and cost less than heretofore attained by conventionalarrangements.

LII

SUMMARY OF THE INVENTION Briefly the regulated power supply describedherein uses a square wave generator the pulse width of which is governedas is known per se by an impedance controlled by the converter output.However, according to the invention this impedance is impressed on thesquare wave generator through a transformer with at least two separatesecondary windings, one connected to the diode bridge output of aconverter output voltage error detector and another connected to thediode bridge output of an overcurrent and/or overvoltage protectingdevice. In this manner physical isolation is provided between thecontrol means and the square wave generator and furthermore, a pluralityof such control means or regulator channels may be provided forsimultaneously acting on the pulse width varying circuitry of the squarewave generator, without galvanic connection between the regulatorchannels themselves.

More particularly the self-regulated DC converter according to theembodiment of the invention described hereafter comprises, in itsoverall configuration, a chopped square wave generator having threeinputs, of which a first is connected to the output of a starter devicewhich stabilizes the DC feed voltage, a second connected through atransformer to the output of an error detector, and a third connectedalso through a transformer to the output of an overcurrent andovervoltage protection device. The output of the chopped square wavegenerator is connected to one input of a power inverter which has asecond input connected through a low-pass filter to the DC feed source,and has two outputs, of which one is connected to the input of arectifier and filter unit and the other to a second input of theprotection device, a third input of the last-mentioned device beingconnected to a second output of the starter device.

With the arrangement described the DC to DC converter functions as aclosed loop regulator on the output voltage, making the pulse ratio ofthe rectangular wave vary in such a manner as to maintain the outputvoltage constant, regardless of variations both of input voltage andload. Inasmuch as the loop is closed by means of transformer coupling,complete isolation of the output circuits with respect to the inputcircuit is achieved. The aforementioned transformer coupling in additionpermits closure of a second regulator loop which becomes effective whenthe alternating current flowing through the power transistors exceeds apredetermined threshold value. In this manner the power transistorsoperate at constant current and are protected from both temporary andpermanent overloads.

It is important to note that this protection, acting directly on thepower transistors, safeguards the more sensitive components of thecircuit and simultaneously controls all the outputs from the secondaryinverter windings; as a result the pro tection mechanism intervenesirrespective of which output is affected by the overload or shortcircuit.

Another protective function assigned to this second regulator loop isthat relating to load overvoltages. If as a consequence of a fault inthe main voltage regulator, the output voltage should rise above apredetermined threshold value, the foregoing regulator loop wouldtransform itself from a current limiter to a voltage limiter,safeguarding the loads connected to the outputs. Because of the absoluteindependence of the two regulator loops, which hereinafter will becalled channels A and B, and which are separated galvanically by thepresence of the transformer, it is reasonable to assume that this powersupply is sufficiently safe for use with sensitive equipment such asused in modern telecommunications systems.

This closed loop output voltage regulator thus presents all the usualadvantages of normal DC to DC converters, in that, because of the ON OFFoperation of the transistors, it operates with satisfactory efficiencyand that it isolates the load from the input circuits. It can be usedfor stepping the voltage down or up.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood from the following description of one embodiment given by wayof example with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram ofa self-regulated DC to DC converteraccording to the invention;

FIG. 2 represents the chopped square wave at the output of the powerinverter;

FIG. 3 is a partially schematic circuit diagram of the converter of FIG.1;

FIG. 4 is the detailed electrical diagram of the chopped wave generatorshown in FIG. 3 in block form only, together with components which serveto produce quenching pulses;

FIGS. 5 and 6 are further diagrams, which serve to explain the operationof the converter of FIG. 1.

As shown in FIG. I, the self-regulated converter according to theinvention comprises a controlled power inverter IF the input of which isconnected to the output of filter Fl fed at IN from a DC source and anoutput of which is connected to the input of a rectifier and filter unitRF having a number of outputs US. One of these outputs is connected tothe input of an error detector and amplifier RE, to one of the inputs ofan overcurrent and overvoltage protection device PS and to one of theinputs ofa starter device DA, a second input of which is connected to asecond output of the filter Fl. A chopped square wave generator GOP hasa first input connected to the output of the error detector andamplifier RE, a second input connected to the output of the protectiondevice PS and a third input connected to the starter device DA. Theoutput of the generator GOP is connected to a second input of the inverter IP which has a second output connected to a third input of theprotection device PS.

The operation of the converter is as follows:

The DC input voltage is fed through the filter FI to the power converterIP which is of the push-pull type and it feeds a chopped square wavevoltage of the form shown in FIG. 2 to the rectifier and output filterunit RF.

The amplitude of this voltage is proportional to the value of thebattery voltage V,,,, and is determined by the transformer ratio of theinverter output transformer. This alternating pulse wave of constantrepetition frequency F I T is generated by the chopped wave generatorGOP. The width of the pulses t depends on the value of the input andoutput voltages, and,

beyond a certain magnitude of input current, also on this latter.

The filter Fl is in the form of a balanced low-pass LC filter and itsfunction is to prevent the high frequency components generated in thesupply source from being passed.

The purpose of the rectifiers and filters RF is to rectify the voltagesfrom the secondary winding of the transformer of the inverter and tofeed the resultant mean value to their respective outputs, as free aspossible from high frequency disturbances.

The square wave generator GOP is a multivibrator similar to that shownby Kadri (US. Pat. No. 3,031,629) and its purpose is to generate thewave form, FIG. 2, necessary for the operation of the power inverter IP.

In this device the width of the pulses generated depends on theinformation which it receives from channels A and B, through the mediumofa control circuit which will be detailed in the description whichfollows. During normal operation of the converter, the width of thepulses is given only by channel A which regulates the output voltage.

The error detector and amplifier RE (channel A) con stitutes the mainregulating unit. It compares the output voltage of the converter with areference voltage, amplifies the difference signal and converts thissignal into a resistance variation which controls the GOP. As will benoted from the description of the Kadri patent, the duration of thepulses generated by the GOP is a function of the resistance in thefeedback loop of the multivibrator. The location of this resistance andthe method of effecting the above-mentioned variation will be discussedlater. Thus, in keeping with wellknown closed loop regulationprinciples, a variation in the output voltage results in a change of theduration of the pulses which opposes the foregoing variation.

The overcurrent and overvoltage protection device PS (channel B) has thedual function of guarding the power transistors against excessivestresses, in case of short circuits or output overloads, and of guardingthe loads against dangerous increases in output voltages, due to abreakdown or malfunction of the main regulator described in thepreceding paragraph.

In this case the input current and output voltage are also compared witha reference level and they act on the width of the pulses of the GOP bymeans of a resistance variation, as in the case of the main regulator ofthe preceding paragraph.

The object of the starter device DA is to power the GOP and for correctoperation this requires a constant voltage supply. During normaloperation this condition is satisfied by using for the GOP thestabilized output voltage of the com verter. During the starting phase,however, this voltage does not exist, or at least it has not yetattained its normal value, and consequently the DC input to theconverter must be used as an energy source. Thus the object of thestarter device is to take a portion of the input power and pass it undervoltage stabilized conditions to the GOP. As soon as the converter is ina steady state condition, this power portion is derived from the outputcircuit.

"Current" channel B is fed continually by the starter device because thecircuit must always be effective, especially when the output voltagedrops or disappears because of an overload. Its consumption under normaloperating conditions is so low as not to influence the overallefficiency of the system to an appreciable extent.

The individual circuits which comprise the self-regulated converter ofFIG. I will now be described in detail with reference to FIGS. 3 and 4.

The power inverter IP is of the push-pull common emitter type andutilizes a pair of transistors l and 2 as switches which are controlledby the pulses furnished by the chopped wave generator GOP and arepresent between the terminals P-0 and R-Q, as indicated in the topcenter portion and the lower right portion of FIG. 3. The common pointof the emitters is connected to the negative input terminal of theconverter by way of resistor 3 and filter inductance 4. Resistor 3 has asensing function and it passes to the input of the overcurrentprotection circuit PS unipolar voltage pulses which are proportional tothe current pulses traversing transistors l and 2. The terminals 1, y of3 are connected to similar terminals 1:, y of the protection device PS(channel B) as indicated in the drawing. Capacitor 5 nullifies theeffects of the continuous voltage component which is present across 3,thereby preventing continuous polarization of the core of transformer 9.The supply circuit for the inverter is completed by way of thecollectors of transistors 1 and 2, the windings 6 and 7 of powertransformer T and the other filter inductance 8. Inductances 4 and 8together with capacitors ll, and 27 form a balanced filter the purposeof which is to attenuate the alternating current components received bythe inverter, thereby substantially keeping these components from beingpassed down the line.

The control pulses of the power inverter I? reach the bases of thetransistors l and 2 by way of speed up units which consist of resistors16 and 17 and capacitors l2 and 15 connected in parallel to the tworesistors, respectively. Resistors I3 and 14 provide shunt paths to thebase-emitter circuits of transistors l and 2 during their OFF condition.Zener diodes l8 and 19 provide surge protection for transistors l and 2during their switching intervals.

The control pulses from the GOP have the form shown in FIG. 5 and differfrom those at the output of the inverter (FIG. 2) by the presence ofquenching pulses (hatched areas) which are essential for the correctturn-off of the power transistors.

These quenching pulses which are superimposed on the form of theoriginal wave produced by the Kadri multivibrator, are generated in thefollowing manner:

Referring to the diagram of the GOP, FIG. 4, the secondary windings 21and 24 are associated with the master inverter and form across theirends a square wave voltage as shown in FIG. 6a.

The windings 22 and 23 which are associated with the slave inverter formacross their ends a voltage which, because of the phase displacementaction of the two inverters taken together, has the form shown in FIG.6b. Considering the series connection of windings 23 and 24 and theirpolarity, the wave form of the resulting voltage between points P and Sis shown in FIG. 6c. Similar reasoning applies to windings 21 and 22 asregards the voltage between points R and S, which is phase displaced byT/2 with respect to V From the polarity of windings 25 and 26 it will bereadily appreciated that the voltage between points U and V, provided bythe connection of these two windings, will have the form shown in FIG.6d.

Diode bridge 28, 29, 30, 3I rectifies the pulses of FIG. 6acorresponding to the hatched areas of FIG. 5 and feeds them to the endsof resistor 32 to provide voltage Vsq, FIG. 62. The composite controlpulse resulting from the sum of the voltages Vps V V is shown in FIG.6f; it coincides with that already shown in FIG. 5.

In addition to windings 6 and 7, power transformer T also compriseswindings 33, 34, 35 and 36. The first two, together with diodes 37 and38, filter inductance 39 and capacitor 40, provide an auxiliarystabilized voltage for the operation of the regulating and controlcircuits. windings 35 and 36 supply the power to the load. As theinverter is of the driven type, the core of T, operates with a fluxdensity sufficiently removed from saturation level and this permitshysteresis losses to be reduced.

Considering now the rectifier and filter unit RF, windings 36 and 35supply the output loads, in a conventional way, through diodes 43 and44, inductance 45 and capacitor 46 for the positive output and similarlythrough diodes 41, 42, inductance 47 and capacitor 48 for the negativeoutput. As previously indicated, these filters are of the mean valuetype to allow use of the regulating arrangement described herein. In thepractical version of the present supply device, the regulator loop isderived from the positive output and the corresponding terminal isindicated in the diagram of FIG. 3 by the reference Z.

As stated above, the chopped square wave generator GOP is amultivibrator similar to that shown by Kadri. This circuit generates twosquare waves displaced in time and the combination of these two squarewaves yields a wave of the form shown in FIG. 6c.

The two square waves are generated by two inverters of which one, whichwill be referred to as the master inverter, can be of the conventionaltype, with its oscillation frequency determined by the saturation of thetransformer. Referring to FIG. 4, this master inverter consists oftransformer 49, transistors 50 and 5t, diodes 52, 53, 54, resistors 55,56, 57 and capacitor 58.

The other inverter which will be referred to as the slave inverterconsists of transfonner 60, transistors 65, 66, diodes 67, 68, 69, 70resistors 7], 72 and capacitor 60'. It receives its control signal fromwinding 59 of the first inverter, which determines the operatingfrequency of both.

A peculiarity of this slave inverter is that the phase of the squarewave produced by it across the windings of transformer 60, is delayed bya time 1,, (FIG. 6) with respect to the phase of the master, because ofthe inclusion of a displacement network consisting of the aforementionedwinding 59, windings 73 and 74 which act at the same time as basedrivers for transistors 65 and 66 capacitor 60' and the equivalentresistance across winding 61, the latter forming part of the transformer62. The foregoing displacement is rendered variable by the differentvalues which this equivalent resistance may assume, in consequence ofthe load transferred from windings 63, 64, which, as will be seenfromFlGS. 3 and 4, are connected to channels A and B respectively, ofthe described regulator device.

The circuit according to the present invention distinguishessignificantly from the Kadri circuit in that in the former the directconnection between the control circuit and displacement network has beenreplaced by a transformer with three windings. At least for applicationsof interest in connection with the present invention, this has thefollowing advantages:

a. it provides galvanic isolation between the regulator means and themultivibrators.

b. it permits simultaneous action of two or more regulator channels onthe same displacement network, without creating galvanic connectionsbetween the regulators themselves.

The variable resistance which is transferred to the displacement networkconsists of resistor 77 in parallel with the resistance presented bytransistor 75 and possibly also transistor 76, in case the latter iscalled into action due to the presence of an overcurrent or overvoltage.

Transistors 75 and 76, as, incidentally, also the control transistor ofthe Kadri circuit, function as constant current loads with the magnitudeof these loads determined by the value of the base current impressed onthem.

It should be emphasized that the value of the displacement t, dependsalso on other quantities, such as the capacity C of capacitor 60 shownin FIG. 4, which however remain fixed during the operation of thecircuit.

As the supply voltage which conditions the oscillation frequency of themaster inverter is one of these quantities, it is clear at this pointwhy care was taken to provide the GOP with a stabilized supply voltage,which, as shown in FIGS. 3 and 4, is taken from the points M and N ofthe starter device. This supply voltage undergoes further filtering byway of inductor 78 and capacitor 79, at the power input to the GOP (FIG.4).

The error detector and amplifier RE (channel A) will now be described indetail.

A part of the output voltage, which is derived by means of voltagedivider 80, 81, 82, is compared with the reference voltage across thezener diode 83. This latter is fed from resistor 89.

The difference between these two voltages, which is impressed on thedifferential amplifier consisting of transistors 84 and 85 and resistors86, 87, 88 is amplified and eventually used to control transistor 75.

power inverter. As the mean value of the rectified output volt- 1 age isproportional to this width, it follows that the output volt age drops.The negative feedback necessary for the operation of the regulator loopis provided in this manner.

Turning now to the description of the overcurrent and overvoltageprotection device (channel B) the supply current to the power inverteris measured by the sensor 3, FIG. 3, and is passed in the form of avoltage proportional to this current to the input of channel B betweenpoints C and D. This voltage is rectified and doubled by means of diodes90 and 9! and capacitors 92 and 93 and then amplified by transistor 94.

The amplified voltage across the output terminals of resistor 95 is thencompared with the voltage of zener diode 96 and fed to the input of thetransistor 76.

This latter acts in a manner similar to transistor 75 of channel A. Anincrease in current beyond the threshold value fixed by zener 96, causestransistor 76 to conduct and hence leads to a reduction of the pulsewidth. The negative feedback so provided is such as to maintain the peakvalve of the current pulses traversing transistors 1 and 2 constant,thereby protecting them against possible overloads and short circuits.This action is added to that of diode 97 and resistor 98 in thefollowing manner:

In the presence of strong overloads or short circuits the voltage atpoint T falls below the threshold limit fixed by zener 96 and by thedrop in voltage across resistor 99, because of which diode 97 starts toconduct.

In consequence the base current of transistor 76 undergoes a markedincrease, which rapidly causes the power transistors to operate with avery reduced phase angle.

The operation of zener diode 101 and resistor 100 is different. In thecase of an output overvoltage, which is automatically reflected as anovervoltage at point M, the zener diode 101 conducts and produces avoltage across resistor 10!] such as to make transistors 94 and 76conduct strongly. The operation at this point is similar to that ofoverload protection and the output voltage remains fixed at the safetylevel determined by zener 101.

The object of this overvoltage protection is to intervene when theoutput voltage, because of the effect of a fault in some component ofchannel A, rises to levels which are dangerous for the equipment to besupplied.

The starter device consists of resistor 102, zener diode 103, transistor104 and diodes 105 and 106.

Transistor 104 operates as an emitter-follower stabilizer and produces astabilized voltage between points D and N which is slightly less thanthat present between T and N under normal operating conditions.

initially the power necessary for starting the supply device comes fromthe foregoing stabilizer and directly supplies channel B from point D,while the GOP receives power from point M by way of diode 105. When theoutput voltages have reached their normal value point T is at a voltagewhich, as stated, is greater than that of point D', because of whichdiode 105 becomes non-conductive and diode I06 begins to operateinstead, In this case the current required by the GOP is provided by thepower inverter and the power loss due to dissipation in transistor [04is eliminated. A small current, however, remains in transistor 104, thiscurrent being due to the powering of channel B.

What is claimed is 1. A self-regulated DC to DC converter of the type inwhich the width of the pulses produced by a square wave generatorincluding two inverters is governed by an impedance on the control sideof the generator, the magnitude of which is varied under the control ofthe converter output, said impedance forming part of an RC-type phasedisplacement network interposed between said inverters; said convertercomprising:

a transformer having a primary winding connected to said control side ofthe generator and having at least two secondary windings each having theoutput path of a transistor connected thereacross by way of a diodebridge;

error detecting apparatus for impressing a first DC control voltage onthe input path of one of said transistors, said first voltage varying asa continuous function of the converter output voltage;

and protecting apparatus for impressing a second DC control voltage onthe input path of the other transistor, said second voltage depending onsaid converter being subjected to excessive electrical conditions;

whereby the width of the pulses produced by said generator is governedby impedance variations in said primary winding which are due tovariations of both said first and second DC voltages and, hence, of thecurrents fiowin g in said transistor output paths. and whereby physicalisolation is insured between both said apparatus and said square wavegenerator and hence between the output and the input of said converter,

2. A self-regulated DC to DC converter as claimed in claim i, whichcomprises a transistor-type power inverter controlled by the output ofsaid square wave generator and having an output transformer with anon-saturable magnetic core.

3. A self-regulated DC to DC converter as claimed in claim 2, whereinthere are provided means for deriving from said power inverter byinductive coupling a voltage proportional to the power inverter current,and wherein said protecting apparatus comprises overload protectingmeans for deriving said second DC control voltage from said proportionalvoltage.

4. A self-regulated DC to DC converter as claimed in claim 3, whereinsaid overload protecting means includes circuit elements for doublerrectifying said proportional voltage and comparing the rectified productwith a reference voltage.

5. A self-regulated DC to DC converter as claimed in claim 2, whereinthe output transformer of said power inverter has first and secondphysically isolated secondary windings from which two DC sources,regulated by said converter, are derived for supplying the converteroutput and powering the square wave generator, respectively.

6. A self-regulated DC to DC converter as claimed in claim 5, whereinsaid protecting apparatus further comprises means including a diode forcomparing the voltage of the last-mentioned regulated DC source with areference voltage, said diode being poled so as to become conductivewhen the voltage of said DC source, as a result of an overload, dropsbelow a predetermined level, thereby causing a marked increase in saidsecond DC control voltage.

7. A self-regulated DC to DC converter as claimed in claim 5, whereinsaid protecting apparatus comprises over-voltage protecting meansincluding a zener diode, said zener diode being connected to one side ofthe square wave generator power supply and becoming conductive to bringabout a marked increase of said second DC control voltage when thesquare wave generator supply voltage exceeds a predetermined level.

8. A self-regulated DC to DC converter as claimed in claim 5, whichcomprises a starting circuit including preregulating means and tworectifier elements, the first of said elements being connected betweenone side of said square wave generator power supply and saidpreregulating means and the second of said elements being connectedbetween said one side of said square wave generator power supply andsaid last-mentioned regulated DC source, said two rectifier elementsbeing poled so that initially said first element is conductive and saidsecond element non-conductive whereby said square wave generator isinitially powered by said preregulating means, and that when the voltageof said regulated DC source reaches its nominal value said first elementbecomes non-conductive and dividual control electrodes of said twoswitching transistors and a common return circuit; a resistor seriallyinterposed in said return circuit; interconnected auxiliary windings onsaid two transformers respectively; and a rectifier bridge having itsinput connected to said auxiliary windings and its output connectedacross said resistor; whereby quenching pulses minimizing the switchinglosses in said switching transistors are added to the aforementionedcombined pulses.

I. t Q i

1. A self-regulated DC to DC converter of the type in which the width ofthe pulses produced by a square wave generator including two invertersis governed by an impedance on the control side of the generator, themagnitude of which is varied under the control of the converter output,said impedance forming part of an RCtype phase displacement networkinterposed between said inverters; said converter comprising: atransformer having a primary winding connected to said control side ofthe generator and having at least two secondary windings each having theoutput path of a transistor connected thereacross by way of a diodebridge; error detecting apparatus for impressing a first DC controlvoltage on the input path of one of said transistors, said first voltagevarying as a continuous function of the converter output voltage; andprotecting apparatus for impressing a second DC control voltage on theinput path of the other transistor, said second voltage depending onsaid converter being subjected to excessive electrical conditions;whereby the width of the pulses produced by said generator is governedby impedance variations in said primary winding which are due tovariations of both said first and second DC voltages and, hence, of thecurrents flowing in said transistor output paths, and whereby physicalisolation is insured between both said apparatus and said square wavegenerator and hence between the output and the input of said converter.2. A self-regulated DC to DC converter as claimed in claim 1, whichcomprises a transistor-type power inverter controlled by the output ofsaid square wave generator and having an output transformer with anon-saturable magnetIc core.
 3. A self-regulated DC to DC converter asclaimed in claim 2, wherein there are provided means for deriving fromsaid power inverter by inductive coupling a voltage proportional to thepower inverter current, and wherein said protecting apparatus comprisesoverload protecting means for deriving said second DC control voltagefrom said proportional voltage.
 4. A self-regulated DC to DC converteras claimed in claim 3, wherein said overload protecting means includescircuit elements for doubler rectifying said proportional voltage andcomparing the rectified product with a reference voltage.
 5. Aself-regulated DC to DC converter as claimed in claim 2, wherein theoutput transformer of said power inverter has first and secondphysically isolated secondary windings from which two DC sources,regulated by said converter, are derived for supplying the converteroutput and powering the square wave generator, respectively.
 6. Aself-regulated DC to DC converter as claimed in claim 5, wherein saidprotecting apparatus further comprises means including a diode forcomparing the voltage of the last-mentioned regulated DC source with areference voltage, said diode being poled so as to become conductivewhen the voltage of said DC source, as a result of an overload, dropsbelow a predetermined level, thereby causing a marked increase in saidsecond DC control voltage.
 7. A self-regulated DC to DC converter asclaimed in claim 5, wherein said protecting apparatus comprisesover-voltage protecting means including a zener diode, said zener diodebeing connected to one side of the square wave generator power supplyand becoming conductive to bring about a marked increase of said secondDC control voltage when the square wave generator supply voltage exceedsa predetermined level.
 8. A self-regulated DC to DC converter as claimedin claim 5, which comprises a starting circuit including preregulatingmeans and two rectifier elements, the first of said elements beingconnected between one side of said square wave generator power supplyand said preregulating means and the second of said elements beingconnected between said one side of said square wave generator powersupply and said last-mentioned regulated DC source, said two rectifierelements being poled so that initially said first element is conductiveand said second element non-conductive whereby said square wavegenerator is initially powered by said preregulating means, and thatwhen the voltage of said regulated DC source reaches its nominal valuesaid first element becomes non-conductive and said second elementconductive whereby the powering of said square wave generator is takenover by said regulated DC source.
 9. A self-regulated DC to DC convertercomprising: a square wave generator of the type which includes twoinverters each having a transformer and in which the phase between thepulses produced by said two inverters is varied under the control of theconverter output voltage; a power inverter having two push-pullconnected switching transistors, the combined pulses produced by thefirst-mentioned two inverters being impressed between the individualcontrol electrodes of said two switching transistors and a common returncircuit; a resistor serially interposed in said return circuit;interconnected auxiliary windings on said two transformers respectively;and a rectifier bridge having its input connected to said auxiliarywindings and its output connected across said resistor; wherebyquenching pulses minimizing the switching losses in said switchingtransistors are added to the aforementioned combined pulses.